By Jocelyn Noveck | Associated Press

“How did we find this place, anyway?”

Wait, where have we heard that before?

Actually, it’s a familiar line in a new genre of thriller that’s emerged the last few years: Airbnb horror films. There are a few constants: Attractive millennials, lots of good booze, a gorgeous weekend rental. And then something scary happens, and finally someone says: “Wait, how did we find this place?” Oops! Suddenly everyone realizes that the house actually chose THEM. By then, it’s too late to escape.

By the time “Happily,” an ambitious, sometimes compelling but wildly uneven debut feature by BenDavid Grabinski, gets to this point, it’s already lost us a bit, and started feeling like yet another slick hipster horror film — you know, for those who like their millennials attractive and their chef-quality kitchen appliances even more so. That’s too bad, because the setup — the first act, as it were — is delicious.

“Happily” begins with this question: Does the “law of diminishing returns” apply in a biological sense to marriages, or can a couple remain as infatuated with each other as the day they met, year after year? The test case is Tom (Joel McHale) and Janet (Kerry Bishe), an absurdly attractive couple still in the honeymoon phase of their marriage. These two can’t go to a party without having a steamy tryst in the bathroom.

Their friends are annoyed — and not just because the bathroom is tied up. They’re annoyed because this honeymoon phase has lasted 14 years and shows no sign of abating.

“They’re going home for Round 2,” quips their jaded friend Karen at one such party, voice dripping with jealousy. She and husband Val do not have the same problem. Another “problem” Janet and Tom have is that they always forgive each other for any transgression. Genuine forgiveness, not the tired cease-fire some couples fall into.

A few nights later at dinner, Karen and Val drop Janet and Tom from an upcoming weekend vacation and return their deposit. “Everyone hates you,” Karen explains. “You never even fight!” The couple protests, but they’re proving Karen’s point by engaging in hanky-panky under the table.

Shaken, they go home and tell each other: “We are not the weird ones. They are.” Soon though, they have worse problems. A creepy older man (Stephen Root) shows up at their house with a briefcase, explaining that he works for “the city” and they have a major malfunction — a rare dual malfunction — in that the law of diminishing returns inexplicably doesn’t apply to them. It all needs to be corrected with an injection. Once injected, they “will finally be totally normal.” And they don’t have a choice.

Not to reveal too much, but violence ensues. Now it’s a tale of a dead body. And a weekend getaway with eight friends — because they’re suddenly invited again. And this time, they have a terrible secret.

The second act turns into something more muddled and less interesting. It doesn’t help that none of these friends are even remotely appealing or compelling. Not nervous Patricia (Natalie Morales), who made the house booking, and her odd husband Donald (Jon Daly), not jealous Karen (Natalie Zea) who wants to sleep with Tom to prove he’s getting tired of his wife, and her always-annoyed husband Val (Paul Scheer), not nasty, puzzle-obsessed Richard (Breckin Meyer) and his poor mistreated girlfriend Gretel (Charlyne Yi). Only Carla (Shannon Woodward) and her partner Maude (Kirby Howell-Baptiste) seem the least bit worth spending a weekend with.

Why are they all even there? Well, there’s a frightening reason they ended up in this particular house, which emerges in due course. But what we’re really asking is, why are these people even together? Nobody seems to enjoy each other or have any fun at all. OK, there’s the tense atmosphere caused by the unexpected murder and all, but there’s no sense these people EVER enjoyed each other.

Without spoiling any secrets, the film progresses in horror-film mode before, in its third act, tying things up in a somewhat clever, unexpected way. By then, though, you may have given up on this group.

Call it the law of diminishing returns.

“Happily”

2 stars out of 4

Rating: R (for sexual content, language throughout and brief violence)

Running time: 96 minutes